Interleukin 12 (IL-12), formerly called natural killer cell stimulatory factor (Kobayashi et al. (1989) J. Exp. Med. 170, 827-845) and cytotoxic lymphocyte maturation factor (Stern et al. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 6808-6812), has potent anti-tumor and antimetastatic activity in several murine tumor models (Brunda et al. (1993) J. Exp. Med. 178, 1223-1230; Nastala et al. (1994) J. Immunol. 153, 1697-1706). Although the mechanism through which IL-12 exerts its anti-tumor effects is not completely understood, it has been shown that IL-12 induces a variety of biological effects on natural killer and T cells in vitro (Manetti et al. (1994) J. Exp. Med. 179, 1273-1283; Wu et al. (1993) J. Immunol. 151, 1938-1949; Tripp et al. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 3725-3729; Seder et al. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 10188-10192; Bloom et al. (1993) J. Immunol. 152, 4242-4254; Cesano et al. (1993) J. Immunol. 151, 2943-2957; Chan et al. (1992) J. Immunol. 148, 92-98). Activation of cytotoxic T lymphocytes by IL-12 is considered crucial in its anti-tumor activity (Brunda et al. (1993) J. Exp. Med. 178, 1223-1230). The IL-12 anti-tumor effect is partially maintained in severe combined immune deficient (SCID) and nude mice, both of which are T cell-deficient, and in CD8*-depleted euthymic mice (Brunda et al. (1993) J. Exp. Med. 178, 1223-1230; O'Toole et al. (1993) J. Immunol. 150, 294A). These results demonstrate that IL-12 has potent in vivo antitumor and antimethastatic effects against murine tumors and demonstrate as well the critical role of CD8.sup.+ T cells in mediating the antitumor effects against subcutaneous tumors.
Interferons (IFNs) are naturally occurring proteins which have antiviral, antiproliferative and immunoregulatory activity. Four distinct classes of interferons are known to exist in humans (Pestka et al. (1987) Ann. Rev. Biochem. 56, 727-777 and Emanuel & Pestka (1993) J. Biol. Chem. 268, 12565-12569). The IFN.alpha. family represents the predominant class of IFNs produced by stimulated peripheral blood leukocytes (Pestka et al., loc. cit.; Havell et al. (1975) Proc. Natl. Acad. Sci. U.S.A. 72, 2185-2187; Cavalieri et al. (1977) Proc. Natl. Acad. Sci. U.S.A. 74, 3287-3291), and lymphoblastoid and myeloblastoid cell lines (Familletti et al. (1981) Antimicrob. Agents. Chemother. 20, 5-9). The antiviral effect of IFN.alpha. is achieved not by a direct influence on the viruses themselves, but by an activity on their target cells in the sense of a protection against the virus infection. The interferons can exert effects on cancer tumors and can influence the immune system of the body on that, for example, they activate macrophages and NK cells and intensify the expression of various immunologically significant constituents of the cell membrane. Details of the preparation of interferon-cDNA and the direct expression thereof, especially in E. coli, have been the subject of many publications. Thus, for example, the preparation of recombinant interferons is known, for example, from Nature 295 (1982), 503-508, Nature 284 (1980), 316-320, Nature 290 (1981), 20-26, Nucleic Acids Res. 8 (1980), 4057-4074, as well as from European Patents Nos. 32134, 43980 and 211 148.
IFN.alpha. has proven to be effective in the treatment of viral infections, e.g. both Hepatitis B and Hepatitis C virus (HBV, HCV) infections, however a significant number of patients do not respond to this cytokine. The ability of IL-12 to promote both Th1 maturation and the enhancement of CTL and NK activity is likely to be critical for its ability to protect against disease in mouse models of viral infections (Orange et al. (1994) J. Immunol. 152, 1253-1264).
The present invention relates to the field of prevention and treatment of infectious diseases using IL-12 in combination with IFN.alpha.. Surprisingly, sub-optimal doses of IL-12 and IFN.alpha. promote effective protection in vitro and in vivo against infectious diseases, especially against viral and parasite infections and intracellular bacterial infections.